Hydrothermal dolomite conglomerations occur from middle to upper Cambrian consistent with the formation from evaporite to dolomite in the northwestern Tarim basin. It has typical saddle crystal morphology, curved crystal and wavy extinction in thin section under crossed-polarized light. Hydrothermal dolomites have higher Ca/Mg values and Sr contents, slightly higher Fe, Mn and Ba contents, and more depleted δ18O values (-10.1~-12.2‰) than those of matrix dolomite (-6.1~-7.8‰). The δ13C values of hydrothermal dolomite in different location are obvious distinct, but are similar to those of related host dolomite. The homogeneous temperatures of primary inclusions in the dolomites are between 90oC and 120oC. The field geometry, petrography and geochemistry of hydrothermal dolomites support that hydrothermal dolomite deposited in fluids from formation water in the silt-crystalline dolomite at fault-controlled setting, and meteoric water under deep circulation. At the beginning of the fault activity, because of pressure difference the formation water began to migrate along the silt-crystalline dolomite. At the same time the anhydrite nodule were dissolved by the fluid, and result in formation of dissolved pores in the dolomite. With the temperature and pressure decreasing, CO2 was released, resulted in the hydrothermal dolomite formed closed to the fault. The formation water in Cambrian silt-crystalline dolomite and may be one of hydrothermal fluid sources, which can dissolute the above carbonate and form reservoir.